The known asynchronous transfer mode (ATM) transmission technique is a modern telecommunications switching technique which is able to switch connections for a wide range of different data types at a wide range of different bit rates. ATM technology provides a flexible form of transmission which allows various types of service traffic data, eg. voice data, video data, or computer generated data to be multiplexed together onto a common physical means of transmission, Currently, several trends are encouraging the widespread introduction of ATM; for example the availability of high speed, low error rate communication links between switching centers, an availability of technology to digitize video and speech, and pressure to reduce operating costs by integrating previously separate telephony and data network. ATM technology allows speech data, video and inter-computer data to be carried across a single communications network. The information carried in each of these services is reduced to digitized strings of numbers which are transmitted across such a communications network from point to point.
Referring to FIG. 1 herein, there is illustrated schematically a pair of known interworking functions 101 and 102 connected by an ATM virtual channel connection (ATM VCC) 103 across an ATM network 100. FIG. 1 illustrates a pair of interworking functions which may be residing at a pair of distinct telecoms resources eg switches, cross connects, comprising processing means and memory means. It will be understood by those skilled in the art that a plurality of interworking functions may be interconnected over a plurality of ATM VCC. Incoming to first interworking function 101 are, for example, a plurality of digitized voice signals multiplexed together using the E1 or T1 mutiplexing systems. First and second interworking functions 101 and 102 are configured to convert data arriving in a plurality of forms and conforming to a plurality of different data standards into a form suitable for transmission over ATM VCC 103 across ATM network 100. Interworking functions 101 and 102 are also configured to perform conversion of data sets across ATM network 100 back into a plurality of different data standards. Each of these incoming channels may carry data at a bit rate of 64 kilobits per second (kbits/s).
There is a financial cost associated with leasing sufficient bandwidth between interworking functions 101 and 102 to transmit a plurality of uncompressed 64 kbits/s channels down a single ATM virtual circuit 103. Therefore, it is known to apply a range of different data compression techniques in order to make more efficient use of the available band width. For example, the known G series data compression techniques can be used to compress streams of data having a bit rate of 64 kbits/s down to bit rates of 40 kbits/s or 8 kbits/s. In addition, speech activity detection (SAD) techniques can be used to suppress pauses in speech data allowing other data to be inserted in the gaps. After compression, there may exist a plurality of compressed 8 kbits/s data to be transferred between interworking functions 101 and 102 across ATM virtual channel 103 via ATM network 100. The plurality of compressed channels of data may represent a plurality of calls between a plurality of users.
Referring to FIG. 2 herein, there is illustrated schematically first and second interworking functions 101, 102 represented as a set of functional layers. Each interworking function comprises the following protocol stack of functional layers:                a Service Specific Conversion Sub-layer (SSCS) 201 into which compressed voiced data are input and output;        a Common Part Sub-layer (CPS) 202;        an ATM Sub-layer 203; and        a Physical Sub-layer 204.        
The set of functional layers 201 to 204 are well known in the art. Each functional layer within the protocol stack is configured to exchange information with the functional layer above it and the functional layer below it. The exchange of information between functional layers within the protocol stack enables each functional layer to provide a service for the functional layer immediately above it. Each functional layer can be considered to exchange information directly with the same functional layer within a protocol stack residing within a distant interworking function across a virtual channel. The exchange of information between interworking functions 101 and 102 is actually effected by the exchange of information across a physical connection between physical sub-layers 204 and 208. Compressed voice data which are input into the interworking function 101 by the Service Specific Conversion Sub-layer 201 are transferred across an ATM network to the remote second interworking function 102 which comprises a similar protocol stack to interworking function 101. The output of Service Specific Conversion Sub-layer 205 of first interworking function 102 is compressed voice data.
Whilst transmission of compressed voice data is shown in one direction in FIG. 1, the protocol stacks are bi-directional and transmission can occur in both directions between first and second interworking functions 101, 102.
In FIG. 2 herein there is illustrated a single Service Specific Conversion Sub-layer 201 associated with interworking function 101. In a real network there may be a plurality of such Service Specific Conversion Sub-layers receiving a plurality of compressed voice data channels. The Common Part Sub-layer 202 is configured to receive a plurality of compressed voice data channels from the plurality of Service Specific Conversion Sub-Layers and multiplex the plurality of compressed voice data channels together. The combination of Service Specific Conversion Sub-layers 201 and Common Part Sub-layer 202 in interworking function 101 are also known in the prior art as ATM Adaption Layer Type 2 (AAL2).
In order to make best use of the available band width over a single ATM. VCC it is known to have a plurality of channels carrying a plurality of, for example, compressed voice data channels. In order to be able to carry a plurality of separate streams of digital data across a network using a plurality of ATM AAL 2 channels over a single ATM virtual channel connection (VCC), there is a need to be able to identify individual ATM AAL 2 channels at both interworking functions.